1. Field of the Invention
The present invention relates to a projection lens apparatus for allowing a source light to be made incident in an image display apparatus so that an overhead projection of the reflected light or the transmitted light is performed on a projection screen, and also relates to a projection type image display apparatus equipped with the projection lens apparatus.
2. Description of the Prior Art
The main stream of a projection type image display apparatus has been a type that performs an overhead projection of an image formed in a CRT. In order to improve the efficiency of light usage, and reduce the size and the weight of the apparatus, a projection type image display equipped with a liquid crystal display element and a micro mirror display element (hereinafter, referred to as a DMD element) instead of a CRT has been recently developed. This projection type image display apparatus is configured such that a source light is modulated by transmitting or reflecting it at each pixel in accordance with video signals applied to the above display element, thereby allowing a projection lens apparatus to perform a overhead projection and thus form an image.
Although the image display apparatus equipped with a CRT is capable of electrically compensating distortion aberration of a projection image, the image display apparatus equipped with a liquid crystal display element or a DMD element is not. For that reason, the latter apparatus is required to optically compensate the distortion aberration.
A projection type lens apparatus using liquid display elements also has been developed as set forth, e.g., in JP-A-11-149041 specification, however, it has a drawback of a short back focus of about 20 mm.
In view of the above problems, it is an object of the present invention to provide a projection lens apparatus for forming an image with high resolution by satisfactorily compensating various kinds of aberration, while securing an appropriate back focus so that another optical component is arranged between the projection lens apparatus and an image display element such as a liquid crystal display element and a DMD element.
In order to attain the above described object, a projection lens apparatus according to the present invention is designed to satisfy the following conditions (1) to (4), in which a first lens group having a negative refracting power, a second lens group having a positive refracting power, and a third lens group having a positive refracting power are arranged from a projection screen side (a light emission side) to an image display element side (a light incidence side) in this order.
1.4 less than |f1|/fxe2x80x83xe2x80x83(1) 
2.8 less than f3/fxe2x80x83xe2x80x83(2) 
3.6 less than fb/fxe2x80x83xe2x80x83(3) 
0.1 less than |f1/f23| less than 0.3xe2x80x83xe2x80x83(4) 
here, f, f1, and f3 represent focal lengths of a whole lens system, a first lens group, and a third lens group, respectively, and f23 represents a combined focal length of a second and third lens groups, and further fb represents a back focus of the whole lens system.
Power distribution of the first to third lens groups as defined above allows the projection lens apparatus to compensate satisfactorily various kinds of aberration, while the compactness and the wide-angle of the apparatus are achieved.
The conditions (1) and (2) are set to reduce chromatic aberration of magnification by controlling powers of the first and third lens groups which are to have a maximum off-axis ray height. It becomes difficult to compensate the chromatic aberration of magnification, when |f1|/f and f3/f become equal to or fall below the respectively corresponding lower limits.
The condition (3) is set to secure an appropriate back focus. When fb/f becomes equal to or falls below its lower limit, the back focus becomes smaller, accordingly making it difficult to arrange an optical component between the lens apparatus and the liquid crystal image display such as the liquid crystal display element and the DMD element.
The condition (4) is set to secure an appropriate back focus and to reduce distortion aberration. When |f1/f23| becomes equal to or exceeds its upper limit, positive powers of the second and third lens groups increase, accordingly making it difficult to secure an appropriate back focus. On the other hand, a negative power of the first lens group increases, thereby causing negative distortion aberration to increase, when |f1/f23| becomes equal to or falls below its lower limit.
The projection lens apparatus according to the present invention also may allow the first, second and third lens groups to have aspherical surfaces. If every lens group has an aspherical surface thereat, the first and third lens groups compensate aberration such as distortion aberration and chromatic aberration of magnification on the aspherical surfaces thereof, and the second lens group compensates aberration of an on-axis ray on its aspherical surface. This configuration permits the lens apparatus to have more versatility of possible compensation of aberration, accordingly to perform further compensation of aberration.
In particular, using an aspherical surface as an emission surface of a third lens of the third lens group, having a large off-axis ray height, serves to effectively compensate aberration of an off-axis ray. Using an aspherical surface also as an incidence surface of a second lens of the second lens group serves to most effectively compensate aberration of an on-axis ray.
Using a plastic lens as a first lens of the first lens group, having a largest diameter among lenses of the projection lens apparatus according to the present invention, allows the apparatus according to the present invention to reduce its weight. However, since the plastic lens changes in shape according to changes in temperature and humidity, the lens characteristics thereof such as lens focus and MTF deteriorates, when the plastic lens has a large power. The condition below is intended to control the power of the plastic lens so that the lens characteristics do not deteriorate even when the plastic lens changes in shape according to changes in temperature and humidity.
7 less than |fL1/f|xe2x80x83xe2x80x83(5) 
A typical plastic lens has poor accuracy in shape and large deviations in shape accuracy compared to a glass lens. To suppress the deterioration of the plastic lens having deviations in shape accuracy, a ray is made incident on the plastic lens, in a direction substantially perpendicular to the plastic lens, serving as the first lens of the first lens group in the projection lens apparatus according to the present invention. This configuration allows the plastic lens to have a larger tolerance in shape and enhance its manufacturability.
Achieving the compactness and the wide-angle of the projection lens apparatus is required to reduce the size of the project type image display apparatus. Decentering the optical axis of the lens apparatus against the center of the projection screen allows the projection type image display apparatus to be downsized further. Decentering the projection lens apparatus against the center of the projection screen requires decentering a Fresnel screen against the projection screen.
When the optical axis of the projection lens apparatus is decentered against the center of the projection screen, ratios of peripheral light quantities at the upper and lower parts of the screen are generally different because of different distances from the optical axis of the projection lens apparatus to the upper and lower parts of the screen. In the projection type image display apparatus according to the present invention, difference in ratios of peripheral light quantities at the upper and lower parts of the screen is designed to be 10% or less so that brightness at the upper and lower parts of the screen is substantially equal.